Turbine abradable air seal system

ABSTRACT

An air scal system for a rotor blade assembly of a gas turbine engine includes a substrate. An optional ceramic interlayer may be disposed on an optional bond coat deposited on the substrate. An erosion resistant thermal barrier coating (E-TBC) layer is disposed on the ceramic interlayer (if present) or on the bond coat, or on the substrate. An abradable layer is disposed on the erosion resistant thermal barrier coating (E-TBC) layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. patentapplication Ser. No. 15/026,755 filed Apr. 1, 2016 which is a NationalStage application of PCT/US2014/056051 filed on Sep. 17, 2014 whichclaims the benefit of and priority to U.S. Provisional PatentApplication No. 61/885,774 filed Oct. 2, 2013, all of which areincorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present disclosure relates to blade assemblies, and moreparticularly to air seal systems for a rotor blade assembly of a gasturbine engine, for example.

2. Description of Related Art

Air seals are formed between various components of gas turbine engines,such as between rotating turbine blades and the inner surface of aturbine casing. In this regard, some air seals are provided as anabradable air seal that incorporates an abradable material affixed tothe inner surface of the casing. The abradable material is contacted andabraded by the rotating blade tips of the turbine blades duringoperation.

Other air seals are provided as wear resistant seals. Wear resistantseals also employ materials affixed to the inner surface of the casing.Such materials, however, are selected for erosion resistance, oxidationresistance and/or thermal protection, for example.

Selection of materials for forming such an air seal typically involves acompromise between resistance to wear by erosion, abradability by bladetips, spallation resistance and environment durability. Exemplarymaterials are set forth in U.S. Pat. Nos. 4,936,745 and 5,780,171, whichare incorporated by reference herein in their entireties. Additionally,each of U.S. Pat. Nos. 6,284,323; 7,662,489; 8,343,587; 8,506,243;8,535,783; and U.S. Patent Application Publication Nos. 2008/0138658 and2010/0098865 is incorporated by reference herein in its entirety.Notably, materials with higher abradability tend to suffer from excesserosion, while less abradable materials tend to be susceptible tospallation. Erosion, spallation, and the like can reduce a seal'seffectiveness, which can have a negative impact on thrust specific fuelconsumption (TSFC).

Such conventional methods and systems have generally been consideredsatisfactory for their intended purpose. However, there is still a needin the art for air seal systems that allow for improved lifetime thrustspecific fuel consumption. There also remains a need in the art for suchsystems that are easy to make and use. The present disclosure provides asolution for these problems.

SUMMARY OF THE INVENTION

An air seal system for a rotor blade assembly of a gas turbine engineincludes a substrate having a bond coat thereon. An erosion resistantthermal barrier coating (E-TBC) layer is disposed inboard of the bondcoat. An abradable layer is disposed inboard of the erosion resistantthermal barrier coating (E-TBC) layer.

In certain embodiments, the bond coat is deposited onto the preparedsubstrate. The bond coat layer can be between 0.5 to 18 mils (1.27×10⁻⁵to 4.572×10⁻⁴ meters) thick. In certain embodiments, the bond coat ismetallic having a thickness of 1.27×10⁻⁴ to 2.286×10⁻⁴ meters (5 to 9mils). The substrate layer can be metallic. An optional ceramicinterlayer can be disposed between the bond coat and the erosionresistant thermal barrier coating (E-TBC) layer. In certain embodiments,the interlayer can be deposited inboard of the bond coat at an elevatedsubstrate temperature. The ceramic interlayer can be 2.54×10⁻⁶ to7.62×10⁻⁵ meters (0.1 to 3.0 mils) thick. For example, the ceramicinterlayer can be of 7 weight % yttria stabilized zirconia (7YSZ) atless than or equal to 6% porosity.

In certain embodiments, the erosion resistant thermal barrier coating(E-TBC) layer can be deposited inboard of the ceramic interlayer. Theerosion resistant thermal barrier coating (E-TBC) layer can be 2.54×10-5to 3.81×10-4 meters (1.0 to 15 mils) thick. In certain embodiments, theabradable layer can be a porous layer having a thickness of 5.0 to 50mils (1.27×10⁻⁴ to 1.27×10⁻³ meters). For example, 23% porous 7YSZ,8-15% porous 20-60 wt % GdZrOx, or any other suitable material can beused for the abradable layer.

A transition layer can be formed between each of the ceramic layers,e.g., between the optional ceramic interlayer and the thermal barriercoating as well as between the erosion resistant thermal barrier coating(E-TBC) layer and the abradable layer. The transition layers can have amixture of properties of the adjacent layers. The transition layers caneach have a thickness of 0 to 2.54×10⁻⁴ meters (0 to 10 mils).

A method for forming an outer air seal system for a rotor blade assemblyof a gas turbine engine includes applying an erosion resistant thermalbarrier coating (E-TBC) layer inboard of a substrate. The method alsoincludes applying an abradable layer inboard of the erosion resistantthermal barrier coating (E-TBC) layer.

A bond coat can be applied onto the substrate prior to applying theE-TBC layer. A ceramic interlayer can be applied onto the bond coat, andthe E-TBC layer can be applied onto the ceramic interlayer. In certainembodiments, an abradable layer is added inboard of the erosionresistant thermal barrier coating (E-TBC) layer. The step of applyingcan include grit blast preparation of the substrate, HVOF (high velocityoxygen fuel) spraying of a metallic bond coat on to the substrate,diffusion heat treating the bond coat and substrate at 1975° F. in aprotective atmosphere, air plasma spraying a ceramic interlayer whilethe part is held at an elevated temperature, air plasma spraying anerosion resistant thermal barrier coating, and then air plasma sprayingan abradable layer.

These and other features of the systems and methods of the subjectdisclosure will become more readily apparent to those skilled in the artfrom the following detailed description of the preferred embodimentstaken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

So that those skilled in the art to which the subject disclosureappertains will readily understand how to make and use the devices andmethods of the subject disclosure without undue experimentation,preferred embodiments thereof will be described in detail herein belowwith reference to certain figures, wherein:

FIG. 1 is a schematic diagram depicting an exemplary embodiment of a gasturbine engine;

FIG. 2 is a schematic diagram depicting a portion of a representativeblade tip in proximity to the casing of FIG. 1, showing detail of theair seal system formed therebetween;

FIG. 3 is a schematic diagram illustrating exemplary portions of the airseal system of FIG. 2, showing the layers which abrade when the turbineengine is in use; and

FIG. 4 is a flow chart illustrating an exemplary embodiment of a methodfor forming the air seal system of FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made to the drawings wherein like referencenumerals identify similar structural features or aspects of the subjectdisclosure. For purposes of explanation and illustration, and notlimitation, a partial view of an exemplary embodiment of a blade outerseal system in accordance with the disclosure is shown in FIG. 1 and isdesignated generally by reference character 200. Other embodiments ofthe air seal system in accordance with the disclosure, or aspectsthereof, are provided in FIGS. 2 and 3, as will be described.

FIG. 1 is a schematic diagram depicting an exemplary embodiment of a gasturbine engine. As shown in FIG. 1, engine 100 is configured as aturbofan that incorporates a fan 102, a compressor section 104, acombustion section 106, and a turbine section 108. Notably, the turbinesection is defined, at least in part by a turbine casing 109, whichforms a portion of the casing of the engine. An air seal 200 (e.g., anabradable air seal) is formed between an inner surface of the turbinecasing and a rotating blade (e.g., blade 111) of the turbine.

As shown in FIG. 2, air seal 200 is an outer air seal located betweenthe rotating blades 111 of the turbine (a portion of one of which isdepicted in FIG. 2) and an inner surface 120 of turbine casing 109. Inthis embodiment, several layers of material are applied to the innersurface of the casing to form the air seal 200. These layers include ametallic substrate 210 having a bond coat 212 thereon. The bond coat 212is metallic having a thickness of 1.27×10⁻⁴ to 2.286×10⁻⁴ meters (5 to 9mils), although it is also contemplated that the bond coat thickness canbe between 0.5 to 18 mils (1.27×10⁻⁵ to 4.572×10⁻⁴ meters) thick.

A ceramic interlayer 214 is disposed inboard of the bond coat 212. Theceramic interlayer 214 is deposited onto the inboard surface of the bondcoat 212 at an elevated substrate temperature and is 2.54×10⁻⁶ to2.54×10⁻⁵ meters (0.1 to 1.0 mils) thick, but it is also contemplatedthat the interlayer 214 can be between 2.54×10⁻⁶ to 7.62×10⁻⁵ meters(0.1 to 3.0 mils) thick. The interlayer 214 provides a tough interfacewith the bond coat 212 within which stresses are dissipated and areduced stress interface with the erosion resistant thermal barriercoating E-TBC layer 216 (described below) results, thus improvingspallation resistance. Interlayer 214 can be of 7YSZ at less than orequal to 6% porosity, for example, or of any other suitable material.

An erosion resistant thermal barrier coating (E-TBC) layer 216 isapplied onto the inboard surface of the ceramic interlayer 214. Theerosion resistant thermal barrier coating (E-TBC) layer 216 provideserosion and spallation resistance and may be deposited at a high powerwith low relative spray torch to part surface speed. The erosionresistant thermal barrier coating (E-TBC) layer 216 can be 3.0 to 15mils (7.62×10-5 to 3.81×10-4 meters) thick. It is also contemplated thatthe thickness can be as low as 1.0 mils (2.54×10-5 meters) and as highas 18 mils (4.572×10-4 meters), or any other suitable thickness.

An abradable layer 220 is disposed onto the inboard surface of theerosion resistant thermal barrier coating (E-TBC) layer 216. Theabradable layer 220 is a porous thermal barrier coating having a balancebetween erosion resistance and the ability to be cut by abrasive bladetips. For example, 23% porous 7YSZ or 8-15% porous 20-60 wt % GdZrOx canbe used for the abradable layer 220. It is also contemplated that anyother suitable ceramic abradable material can be used, for exampleceramics filled with a softer phase such as hexagonal boron nitride(hBN). The abradable layer 220 can be 5.0 to 50 mils (1.27×10-4 to1.27×10-3 meters) thick.

A transition layer 218 is formed between the erosion resistant thermalbarrier coating (E-TBC) layer 216 and the abradable layer 220. Thetransition layer 218 has a mixture of properties of the erosionresistant thermal barrier coating (E-TBC) layer 216 and the abradablelayer 220. The transition layer 218 has a thickness between 0 to2.54×10⁻⁴ meters (0 to 10 mils). It should be noted that transitionlayers can be formed between any of the layers, and that the transitionlayers do not necessarily add to the overall thickness, but may beconsidered to be part of the inner most adjacent layer with an engineframe of reference, for example. The transition layers can each be a 5mil transition from the inner adjacent layer to the next adjacent layer.It is contemplated that the transition layers can be between 0 to2.54×10⁻⁴ meters (0 to 10 mils) thick. For example, one transition layercan be a 5 mil transition from the interlayer to the erosion resistantthermal barrier coating (TBC) layer 216. So, for example, in anapplication with a 1 mil interlayer and 15 mil erosion resistant thermalbarrier coating (E-TBC) layer could be considered to be 1 milinterlayer, 5 mil transition, and a 10 mil erosion resistant thermalbarrier coating (E-TBC).

Referring to FIG. 3, the erosion resistant thermal barrier coating(E-TBC) layer 218 and the abradable layer 220 are shown depicting anexemplary rub path 230 created by the rotating blades 111 of the turbineengine. As shown, the abradable layer 220 is allowed to be removed bythe rotating blades. The erosion resistant thermal barrier coating(E-TBC) layer 216 contributes very little to the volume of material thatthe abrasive tips need to cut because the arc length of the blade pathintersecting with the E-TBC layer is relatively short given the shallowdepth of cut into this layer.

An embodiment of a method 240 for forming an air seal system is shown inthe flow chart of FIG. 4. An exemplary preparation of the substrate,e.g., substrate 210, can include grit blasting the substrate. Thesubstrate can be flat as depicted in FIG. 2, or can include geometricsurface features as described for example in U.S. Pat. No. 8,506,243,which is incorporated by reference herein in its entirety. The bondcoat, e.g., bond coat 212, is an oxidation resistant alloy which may beapplied (at block 242) to the substrate by any of a number of processes.The bond coat alloy is known generically as MCrAlY and may be suppliedin powder form. The powder may be deposited by any of a number ofprocesses, however one that produces a dense (e.g. less than 5%porosity), low oxide (e.g., less than 2% oxygen) coating is suitable,for example. It is also contemplated that cathodic-arc or cat-arc thatcan also be used, e.g., using an MCrAlY ingot and can produce loweroxide and better oxidation resistance. A process that operates withparticle velocity greater than 1000 ft/s (304.8 m/s) is suitable, suchas in high velocity plasma spray, high velocity oxygen fuel, highvelocity air fuel, cold spray and warm spray processes, for example. Thebond coat thickness may be 8 to 12 mils (2.032×10⁻⁴ to 3.048×10⁻⁴meters). The coating and part, e.g. bond coat 212 and substrate 210, maythen be diffusion heat treated to further improve bonding and oxidationresistance. For example, PWA 1386 powder, available from Sulzer Metco(all Sulzer Metco products described herein are available from SulzerMetco (US) Inc. of Westbury, N.Y.), is deposited by high velocity oxygenfuel (HVOF) spraying and is then diffusion heat treated at 1975° F.(1079° C.) for 1 hour, e.g., in a protective atmosphere.

In a suitable setup for deposition of the above described ceramiclayers, a plurality of bond coated substrates are loaded into a hollowcylindrical fixture such that the bond coated surfaces face the innerdiameter of the cylindrical fixture. An auxiliary heat source, such asgas burners, is positioned around the fixture and a means for monitoringand controlling the part temperature is employed. This may include athermocouple and temperature controller for regulating gas flow to thegas burners. A plasma spray torch is positioned in the interior of thecylindrical fixture for depositing the layers. In another exemplarysetup, the parts are insulated and the plasma torch provides the heat.Variable air blower pressure may be employed to limit and control parttemperature in this configuration.

The parts can then be preheated to the desired process temperature andthen the ceramic interlayer, e.g., ceramic interlayer 214, is applied atblock 244. After a preheat time of approximately 10 minutes and theparts are at the temperature set point, which is in one example 1200° F.(648.9° C.), the coating process is begun. Air plasma spraying can beused while the part is held at the elevated temperature. As an example,a Sulzer Metco 9 MB torch is operated at 60 kilowatts with 100 scfh(standard cubic feet per hour) (2.83 standard cubic meters per hour) ofnitrogen and 25 scfh (0.708 standard cubic meters per hour) of hydrogengas flow. A suitable powder is a yttria partially stabilized zirconia(yttria partially stabilized zirconia herein refers to a composition ofabout 12 weight percent or less yttria stabilized zirconia). However, acomposition of between about 6 weight percent and about 20 weightpercent yttria stabilized zirconia may be used. In certain applications,a suitable range between about 7 weight percent and about 12 weightpercent yttria stabilized zirconia may be chosen based on materialstrength. Similarly, other zirconia based compositions can be used, suchas ceria stabilized zirconia, magnesia stabilized zirconia, calciastabilized zirconia, and mixtures thereof may be substituted for theyttria stabilized zirconia. An example of a suitable powder is SulzerMetco 204B NS of ZrO₂ 8Y₂O₃ composition. This can be fed through a #2powder port at 20 g/minute with 12 scfh (0.340 cubic meters per hour) ofnitrogen carrier gas. The parts are arranged on a 30 inch (76.2 cm)diameter fixture which is rotated at 10 rpm, for example, which can beadapted for other sizes of fixtures. The torch can be traversed back andforth in front of the parts at a 2.75 inch (6.99 cm) stand-off distanceand 3 inches (7.62 cm) per minute traverse speed.

The erosion resistant thermal barrier coating (E-TBC) layer, e.g.,erosion resistant thermal barrier coating (E-TBC) layer 216, is appliedonto the ceramic interlayer, e.g., interlayer 214, at block 246. Again,air plasma spraying can be used. The process may begin while the partsare still at elevated temperature or may be conducted as a separatespray event after the parts have cooled. The powder may be the same asthat used for the ceramic interlayer or may be switched to one ofdifferent particle morphology or composition. Suitable compositionsinclude those suitable for the ceramic interlay with the additionaloption of gadolinia stabilized zirconia and other low conductivityceramic materials. Suitable powers for the erosion resistant thermalbarrier coating (E-TBC) layer include compositions disclosed in EuropeanPatent No. EP0992603, which is incorporated by reference herein in itsentirety. In an exemplary process, Sulzer Metco 204B NS powder continuesspraying onto the parts as described above without interruption, whilethe process parameters are gradually changed to those for the erosionresistant thermal barrier coating (E-TBC) layer. This produces a gradedzone, or transition layer, which in this example is 3 to 5 mils(7.62×10⁻⁵ to 1.27×10⁻⁴ meters) thick. Including the graded zone, theerosion resistant thermal barrier coating (E-TBC) layer is 12 to 18 mils(3.048×10⁻⁴ to 4.572×10⁻⁴ meters) thick. While depositing the gradedzone, the exemplary process increases rpm to 80, stand-off to 4 inches(0.1016 meters), traverse rate to 12 inches (0.3048 meters) per minute,powder feed rate to 50 g/min (grams per minute) and reduces torch powerto 35 kW with hydrogen flow rate reduced to 18 scfh (0.510 cubic metersper hour) and carrier gas flow rate to 10 scfh (0.283 cubic meters perhour). Auxiliary heating is also stopped, allowing the part temperatureto passively drop as the process progresses. This process can becontinued at these conditions until the target thickness is achieved forthe erosion resistant thermal barrier coating (E-TBC) layer.

The abradable top layer, e.g., abradable layer 220, is then applied ontothe erosion resistant thermal barrier coating (E-TBC) layer, e.g., usingair plasma spraying, at block 248. This may begin while the parts arestill at elevated temperature following the erosion resistant thermalbarrier coating (E-TBC) layer application or may be conducted as aseparate spray event after the parts have cooled. The powder may be thesame as that used for the erosion resistant thermal barrier coating(E-TBC) layer, or may be switched to one of different particlemorphology or composition. In an exemplary process, parameters aregradually adjusted while the first 5 mils (1.27×10⁻⁴ meters) ofabradable layer is being applied. 40 Wt % gadolinia stabilized zirconiapowder is used at a rate of 40 g/minute and injected into the plasmastream with a carrier gas flow rate of 11 scfh (0.311 cubic meters perhour). Torch stand-off distance is increased to 5 inches (12.7 cm) andtraverse rate to 20 inches (50.8 cm) per minute. Fixture speed isincreased to 100 rpm. Coating application is continued until the finaldesired coating thickness is reached. In this example that is 40 mils(1.016×10⁻³ meters) of abradable top layer, which includes 5 mils(1.27×10⁻⁴ meters) of graded transition much as described above. Whilean exemplary composition for the abradable top layer has been describedabove, any other suitable composition can be used, for example, thecomposition can be 8 Wt % yttria stabilized zirconia blended withapproximately 6 Wt % of methylmethacrylate particles (SM2602 from SulzerMetco).

While described above in the exemplary context of including a bond coaton the substrate, it is possible to use the ceramic layers on a specialbase metal that does not require a bond coat. However it is to beexpected that in typical high temperature applications the bond coatwill be required.

While certain layers are referred to herein as being thermallyinsulative, e.g., thermal barrier coatings, and others are referred toherein as being abradable, it is to be understood that all the ceramiclayers can be considered to be insulating, and all may be considered tobe abradable. Moreover, the erosion resistant aspect, e.g., of theerosion resistant thermal barrier coating (E-TBC), is indicative of thepresence of a layer that has lower erosion resistance such as theabradable layer, since all the ceramic layers can be considered to havesome erosion resistance. For example, abradable layer 220 may be erosionresistant like erosion resistant thermal barrier coating (E-TBC) layer,or may be an abradable or low conductivity layer having higher porosity,less strong bonding between spray particles, and/or a compositioncontaining a higher concentration of stabilizing elements such as Y andGd. So, for example, a coating system in accordance with this disclosurecan include a ceramic coating with arbitrary top layer(s) and atoughened interlayer to help prevent spallation. In another aspect, acoating system in accordance with this disclosure can include anabradable coating with a toughened interlayer to help prevent spallationand an optional erosion resistant middle layer.

While described in the exemplary context of turbine abradable layers, itis also contemplated that systems as disclosed herein can be used as athermal barrier coating (TBC). i.e. in applications without rub. In suchapplications, the erosion resistant thermal barrier coating (E-TBC)layer provides some minimum required durable thermal barrier function,while the inboard layer, e.g., the abradable layer, provides additionalthermal insulation in a less durable form.

The methods and systems of the present disclosure, as described aboveand shown in the drawings, provide for an air seal system with superiorproperties, and may include an optional toughened ceramic interlayerwith the erosion resistant thermal barrier layer for improved lifetimethrust specific fuel consumption. While the apparatus and methods of thesubject disclosure have been shown and described with reference topreferred embodiments, those skilled in the art will readily appreciatethat changes and/or modifications may be made thereto without departingfrom the spirit and scope of the subject disclosure.

What is claimed is:
 1. A seal system for a rotor blade assembly of a gasturbine engine, the seal system comprising: a substrate; a bond coatdisposed on the substrate; an erosion resistant thermal barrier coating(E-TBC) layer inboard of the ceramic layer; and an abradable layerconsisting of porous yttria stabilized zirconia (YSZ) or a ceramicfilled with hexagonal boron nitride and disposed inboard of the erosionresistant thermal barrier coating (E-TBC) layer wherein a transitionlayer is formed between the erosion resistant thermal barrier coating(E-TBC) layer and the abradable layer, the transition layer having amixture of properties of the erosion resistant thermal barrier coating(E-TBC) layer and the abradable layer.
 2. A seal system as recited inclaim 1, wherein the bond coat is metallic having a thickness of1.27×10⁻⁴ to 2.286×10⁻⁴ meters (5 to 9 mils).
 3. A seal system asrecited in claim 1, further comprising a ceramic interlayer inboard ofthe bond coat.
 4. A seal system as recited in claim 3, wherein theceramic interlayer is 2.54×10⁻⁶ to 7.62×10⁻⁵ meters (0.1 to 3.0 mils)thick.
 5. A seal system as recited in claim 1, wherein the abradablelayer is a porous, erosion resistant layer having a thickness of 5 to 50mils (1.27×10⁻⁴ to 1.27×10⁻³ meters).
 6. A seal system as recited inclaim 1, wherein the substrate layer is metallic.
 7. A method forforming an air seal system for a rotor blade assembly of a gas turbineengine comprising: applying a bond coat onto a substrate; applying anerosion resistant thermal barrier coating (E-TBC) layer onto theinterlayer; applying a transition layer inboard of the erosion resistantthermal barrier coating layer (E-TBC); and applying an abradable layerconsisting of porous yttria stabilized zirconia (YSZ) or a ceramicfilled with hexagonal boron nitride inboard of the transition layer andthe erosion resistant thermal barrier coating (E-TBC) layer.
 8. A methodas recited in claim 7, wherein applying the bond coat includes highvelocity oxygen fuel (HVOF) spraying a metallic bond coat material ontothe substrate.
 9. A method as recited in claim 8, wherein applying thebond coat includes diffusion heat treating the bond coat and substrateat 1975° F. (1079° C.) in a protective atmosphere.
 10. A method asrecited in claim 7, further comprising applying a ceramic interlayeronto the bond coat, wherein applying the erosion resistant thermalbarrier coating (E-TBC) layer includes applying the erosion resistantthermal barrier coating (E-TBC) layer onto the interlayer.
 11. A methodas recited in claim 8, wherein applying the abradable layer includes airplasma spraying abradable layer material onto the erosion resistantthermal barrier coating (E-TBC) layer.
 12. A method as recited in claim10, wherein applying the ceramic interlayer includes air plasma sprayinga ceramic interlayer material onto the bond coat while the substrate andbond coat are held at an elevated temperature.
 13. A method as recitedin claim 12, wherein applying the erosion resistant thermal barriercoating (E-TBC) layer includes air plasma spraying erosion resistantthermal barrier coating (E-TBC) layer material onto the ceramicinterlayer.